1. Field of the Invention
This invention relates generally to processes for the fluidized catalytic cracking (FCC) of heavy hydrocarbon streams such as vacuum gas oil and reduced crudes. More specifically, this invention relates generally to processes for the catalytic cracking of heavy hydrocarbon feeds and the sequential recracking of recovered product fractions with independent recovery of separate product streams from a single reactor vessel.
2. Description of the Prior Art
The FCC process uses gas streams to contact finely divided streams of catalyst particles and effects contact between the gas and the particles. The FCC processes, as well as separation devices used therein are fully described in U.S. Pat. Nos. 5,584,985 and 4,792,437, the contents of which are hereby incorporated by reference.
The FCC process is carried out by contacting the starting material whether it be vacuum gas oil, reduced crude, or another source of relatively high boiling hydrocarbons with a catalyst made up of finely divided or particulate solid material. The catalyst is transported in a fluid-like manner by passing gas or vapor through it at sufficient velocity to produce a desired regime of fluid transport. Contact of the oil with the fluidized material catalyzes the cracking reaction. The cracking reaction deposits coke on the catalyst. Coke is comprised of hydrogen and carbon and can include other materials in trace quantities such as sulfur and metals that enter the process with the starting material. Coke interferes with the catalytic activity of the catalyst by blocking active sites on the catalyst surface where the cracking reactions take place. Catalyst is traditionally transferred from a stripper, that removes adsorbed hydrocarbons and gases from catalyst, to a regenerator for purposes of removing the coke by oxidation with an oxygen-containing gas. An inventory of catalyst having a reduced coke content, relative to the catalyst in the stripper, hereinafter referred to as regenerated catalyst, is collected for return to the reaction zone. Oxidizing the coke from the catalyst surface releases a large amount of heat, a portion of which escapes the regenerator with gaseous products of coke oxidation generally referred to as flue gas. The balance of the heat leaves the regenerator with the regenerated catalyst. The fluidized catalyst is continuously circulated from the reaction zone to the regeneration zone and then again to the reaction zone. The fluidized catalyst, as well as providing a catalytic function, acts as a vehicle for the transfer of heat from zone to zone. Catalyst exiting the reaction zone is spoken of as being spent, i.e., partially deactivated by the deposition of coke upon the catalyst. Specific details of the various contact zones, regeneration zones, and stripping zones along with arrangements for conveying the catalyst between the various zones are well known to those skilled in the art.
The FCC unit cracks gas oil or heavier feeds into a broad range of products. Cracked vapors from the FCC reactor enter a separation zone, typically in the form of a main column, that provides a gas stream, a gasoline cut, cycle oil and heavy residual components. The gasoline cut includes both light and heavy gasoline components. Major component of the heavy gasoline fraction comprises heavy single ring aromatics and condensed ring compounds.
The recracking of product fractions from the initially cracked FCC product is well known. Recracking can provide a variety of controlled reactions for shape-selective cracking, dealkylation, dehydrogenation, isomerization, and hydrogen transfer reactions. It is well known to contact the feed fraction and recracked freed fractions in separate reaction zones. The prior art also discloses the use of separate risers for the contacting of a primary feed and a secondary feed. Unconverted material from these reactions become rich in highly refractory multi-ring aromatics which consist essentially of methyl and ethyl alkyl groups. The basicity of the these multi-ring aromatics gives them a high affinity for acid cites that causes them to proceed via condensation and hydrogen transfer-type reactions to form coke. Methods are sought to more efficiently contact the primary and recracked feeds, separate the product components from the catalyst streams, use the available activity in the catalyst to its fullest extent, and improve the overall apparatus arrangement for the primary feed contacting and recycle fraction recontacting.
A number of specific process advantages can be obtained by such an arrangement. Such a process can maximize gasoline production while producing a higher quality gasoline product. Suitable processes will eliminate the presence of high boiling aromatics from FCC gasoline. Alternately, middle distillate production can also be improved where desired.
Particular benefits can be obtained by improvements to the separation of the catalyst from the cracked hydrocarbon streams. The most common method of separating particulate solids from a gas stream uses a cyclonic separation. Cyclonic separators are well known and operate by imparting a tangential velocity to a gases containing entrained solid particles that forces the heavier solids particles outwardly away from the lighter gases for upward withdrawal of gases and downward collection of solids. Cyclonic separators usually comprise relatively small diameter cyclones having a tangential inlet on the outside of a cylindrical vessel that forms the outer housing of the cyclone.
Cyclones for separating particulate material from gaseous materials are well known to those skilled in the art of FCC processing. In the operation of an FCC cyclone tangential entry of the gaseous materials and catalyst creates a spiral flow path that establishes a vortex configuration in the cyclone so that the centripetal acceleration associated with an outer vortex causes catalyst particles to migrate towards the outside of the barrel while the gaseous materials enter an inner vortex for eventual discharge through an upper outlet. The heavier catalyst particles accumulate on the side wall of the cyclone barrel and eventually drop to the bottom of the cyclone and out via an outlet and a dipleg conduit for recycle through the FCC arrangement. Cyclone arrangements and modifications thereto are generally disclosed in U.S. Pat. Nos. 4,670,410 and 2,535,140.
U.S. Pat. Nos. 4,397,738 and 4,482,451, the contents of which are hereby incorporated by reference, disclose an alternate arrangement for cyclonic separation that tangentially discharges a mixture of gases and solid particles from a central conduit into a containment vessel. The containment vessel has a relatively large diameter and generally provides a first separation of solids from gases. This type of arrangement differs from ordinary cyclone arrangements by the discharge of solids from the central conduit and the use of a relatively large diameter vessel as the containment vessel. In these arrangements the initial stage of separation is typically followed by a second more complete separation of solids from gases in a traditional cyclone vessel.
It is an object of this invention to improve the operation of dual riser systems for contacting a heavy FCC feed and a recracked lighter feed fraction using a dual riser reactor arrangement.
It is another object of this invention to recontact an intermediate product fraction from the fluidized catalytic cracking of a primary feed fraction with fresh catalyst and to contact the primary feed with a mixture of spent catalyst from the recontacting zone and regenerated catalyst.
It is a further object of this invention to provide an apparatus for recontacting a recycled FCC product fraction with fresh catalyst while returning the partially spent catalyst for recycle to a primary feed contacting conduit.
Another object of this invention is the recracking of heavy gasoline components at high severity to reduce the end point of FCC gasoline with an improved FCC arrangement.